The present invention relates to an automatic sphygmomanometer and, more particularly, to an automatic sphygmomanometer capable of measuring blood pressure in a deflating process after a cuff pressure is inflated to a value higher than that of the maximum blood pressure of a patient.
A well-known automatic sphygmomanometer technique in measuring maximum and minimum blood pressure is one in which the cuff pressure is inflated to a value higher than that of maximum blood pressure by an electric-powered pump, after which the cuff pressure and cuff pulse wave are measured by a pressure sensor, and presence of Korotkoff sounds (hereinafter referred to as "K sounds") is detected, while the cuff pressure is continuously reduced.
In this method, when the cuff pressure has been inflated to a value higher than that of one's maximum blood pressure, the pneumatic pressure (an air pressure) for generating a cuff pressure is high in comparison with the case where the cuff pressure is low. Accordingly, since the velocity of the air which is released via a nozzle of an exhaust valve connecting to the cuff is increased, and the dynamic pressure becomes large, a rather large amount of torque is required to close the exhaust valve once it has been opened. As a result, a large solenoid, consuming a large amount of electric power or another suitable unit, is needed.
On the other hand, in a sphygmomanometer which needs to be constructed as compact as possible, since the volume of a case is limited, only a driving power source having a small capacity, such as a size AA battery can be used even when a solenoid is used.
Accordingly, in order to suppress the exhaust air velocity caused by high pressure and obtain a constant deflating velocity, a plurality of pneumatic resistors and valves for switching those resistors or a plurality of solenoids having small electric power requirement which are respectively incorporated with the valves can be used. Furthermore, in order to maintain a deflating amount of the cuff pressure, for both cases where the cuff pressure is high and low, the exhaust valve open duration is changed according to the cuff pressure. When the cuff pressure is high, the duration for opening the exhaust valve is shorten, while when the cuff pressure is low, the duration is made longer That is, the duration for opening the exhaust valve is controlled substantially in inverse proportion with respect to the cuff pressure.